1. Field of the Invention
The present invention relates, in general, to a composition for forming a dielectric, a capacitor produced using the composition, and a printed circuit board provided with the capacitor. More particularly, the present invention pertains to a composition for forming a dielectric, which is applied to an embedded capacitor with a relatively high dielectric constant, a capacitor produced using the composition, and a printed circuit board provided with the capacitor.
2. Description of the Prior Art
Generally, a passive element mounted on a printed circuit board (PCB) is regarded as an obstacle in miniaturizing a laminated substrate and in improving a frequency of the laminated substrate. Additionally, a rapid embedment trend and an increase of an I/O number in the semiconductor field make it difficult for a plurality of passive elements to be positioned around active elements because of insufficient space around the active elements.
Furthermore, the increased working frequency of semiconductors leads to the use of a capacitor for decoupling so as to stably supply power to an input terminal. In this regard, the capacitor cannot reduce inductance caused by high frequencies until the capacitor is positioned as close as possible to the input terminal.
With respect to the recent trend of the miniaturization and increased frequency of the PCB, a process of positioning the passive element, for example the capacitor, under an active chip of the laminated substrate, or a technology of forming a high dielectric layer in the PCB or multi-layered PCB, in which the high dielectric layer is used as the capacitor, is suggested, so as to mount the capacitor at a desired position around an active chip.
Particularly, a method of reducing electrical inductance, in which a dielectric layer is formed as an embedded capacitor under an active chip to enable the active chip to be electrically connected to the embedded capacitor while being positioned as close as possible to an input terminal to maximally reduce a length of a wire, has been developed in the art, as indicated by the Sanmina patents, U.S. Pat. Nos. 5,079,069, 5,155,655, 5,161,086, and 5,162,977.
As such, it is required that the dielectric layer for forming the embedded capacitor has a relatively high dielectric constant and a relatively low dielectric loss. An epoxy-based material, in which powdered ferroelectric substance, such as BaTiO3, is dispersed in a thermoplastic or a thermosetting resin, has been used as a material of the dielectric layer, as disclosed in Japanese Pat. Laid-Open Publication Nos. Hei. 5-57851, 5-57852, and 7-9609.
However, the epoxy-based material, containing the thermoplastic or thermosetting resin and the ferroelectric substance powder mixed with each other, is disadvantageous in that the dielectric constant of the embedded capacitor is a relatively low 6 to 22.5 when the embedded capacitor is produced using the epoxy-based material, and that it is difficult to secure a dielectric constant of about 20 or more, preferably 50 or more by increasing an amount of the ferroelectric substance powder.
Dielectric properties of the epoxy-based material, depending on a volume of the ferroelectric substance powder, such as BaTiO3, in the epoxy-based material may be calculated by the Lichtenecker equation. In this regard, the Lichtenecker equation shows that when substances with different dielectric constants are connected to each other in parallel or in series, the dielectric constants correlate to a volume ratio of the substances. In case that the substances are connected in series, a total dielectric constant of the substances is reduced. On the other hand, when the substances are connected in parallel, the total dielectric constant of the substances is increased.
Practically, the substances with the different dielectric constants are not arranged in series nor in parallel, but in a combined manner of series and parallel arrangements on a complex substrate. Hence, an exponential value of the Lichtenecker equation depends on a mixing ratio and a mixing manner (series arrangement, parallel arrangement, and the extent of the mixing of the series and parallel arrangements) of the different substances as shown in FIG. 1. In FIG. 1, Kd′ represents the dielectric constant of a spherical particle (i.e. filler) and Km′ represents the dielectric constant of a medium (i.e. resin) when mixing two (2) ingredients.
For example, when the exponential value, n is −1, all of the substances are arranged in series. On the other hand, when the exponential value, n is +1, all of the substances are arranged in parallel. Practically, when a material powder is randomly dispersed, the exponential value of the Lichtenecker equation gets closer to the series arrangement. Accordingly, it is difficult to improve the dielectric constant.
In other words, in case that a composition contains a resin with the very low dielectric constant, such as epoxy acting as a matrix, and ferroelectric substance powder, even though the dielectric constant of the ferroelectric substance powder is improved, the capacitor is produced using the composition containing the resin and ferroelectric substance powder arranged in series. As a consequence, the total dielectric constant of the capacitor is controlled by the value of epoxy having a low dielectric constant.
Meanwhile, the dielectric constant may be increased by increasing a ratio of the volume of the powder to the volume of the resin. Hence, a volume ratio of the ferroelectric substance powder in the composition must be increased in order to produce a composition a relatively high dielectric constant. However, the volume ratio of the ferroelectric substance powder cannot be easily increased because of technical limitations of the process of producing the PCB. For example, when the volume ratio of the ferroelectric substance powder in the composition is increased, a ceramic constituting the PCB becomes brittle, and thus, the productivity of the PCB is reduced. Accordingly, it is difficult to improve the dielectric constant of the composition while ensuring intrinsic flexibility of the epoxy resin.
As well, a method of adding a filler with excellent conductivity, such as metal powder, to a resin has been suggested to improve a dielectric constant of a material. With respect to this, even though an actual interval between electrodes is not reduced by adding the filler to the resin, the same effect as in the case of reducing an interval between capacitors is achieved, and thus an apparent dielectric constant of the material is increased.
However, when the filler (conductor) with excellent conductivity, such as the metal powder, is added to the resin, dielectric breakdown will likely to occur. In detail, when the metal powder is added to the resin in a predetermined amount or more, a percolation phenomenon occurs, and thus, the material does not act as a dielectric, but as a conductor. Therefore, an amount of the conductive filler, such as the metal powder, added to the resin is limited because of the percolation phenomenon.
In addition, when the conductor, such as the metal powder, is added to the resin, the dielectric loss of the material is undesirably increased because of an eddy current of the material, caused by a variation of the frequency of the PCB. With respect to this, a technology for improving a dielectric constant of a material, in which a metal is added to a resin, is disclosed in Japanese Pat. Laid-Open Publication No. 2002-334612.
An effort has been made to avoid the undesirable percolation phenomenon occurring when the metal powder is added to the resin, in which a insulate dielectric layer is coated on the metal powder. However, this technique is problematic in that even though the percolation between metal particles is suppressed and the dielectric constant is improved, the dielectric loss is relatively high.